CIRP’s study estimates that approximately 63 percent of Amazon customers are now Prime members. The study illustrates why Amazon works so hard to add as many people as possible to Prime. Prime members surveyed spent twice as much on Amazon as non-members — $1,300 per year versus $700.

Amazon doesn’t publicly disclose the official number of Prime members, so observers have to rely on surveys and estimates. For its study, CIRP surveyed 500 U.S.-based Amazon customers who made a purchase from July through September.

But some analysts have called CIRP’s previous findings into question. When CIRP released its last study back in July, estimating Prime membership at 85 million, Moody’s put the number much lower, closer to 50 million. Michael Pachter, a research analyst for Wedbush Securities, told GeekWire that the study was “flat out wrong.” He put Amazon’s Prime membership at about 52.5 million.

Prime is one of the main “pillars” of Amazon’s business. The online retail giant continuously adds new perks for members to induce more customers to sign up; free 2-day shipping, cloud storage, streaming media, and access to exclusive deals are just some of the benefits. Amazon CEO Jeff Bezos said last year that the company’s goal “is to make sure that if you are not a Prime member, you are being irresponsible.”

CIRP found that these perks also help Amazon keep Prime members. Approximately 96 percent of respondents reported that they would “definitely” or “probably” renew their Prime subscriptions.

“Even as Amazon Prime adds late adopters to the ranks of Prime members, loyalty remains high,” said Mike Levin, Partner and Co-Founder of CIRP. “The share of members that indicate they are likely to renew for another year has remained high for several quarters, always comfortably above 90%. In fact, in some quarters, literally no customers in the sample indicated they would definitely not renew their Amazon Prime membership.”

Nat Levy is a staff reporter at Geekwire covering a variety of technology topics, including Microsoft, Amazon, tech startups, and the intersection of technology with real estate, courts and government. Contact him at nat@geekwire.com and follow him on Twitter at @natjlevy.